In recent years, to meet increasingly severe corrosion resistance requirements for automobile bodies, surface treated steel strips in the form of conventional cold rolled steel strips plated with zinc or zinc base alloy are often used. Particularly in the area where corrosion is a serious problem, high bare corrosion resistance is needed in internal strip configurations including internal cavity-defining structures and bends where protective coatings after pressing and body assembly cannot fully cover. Organic composite coated steel strips having chromate and organic coatings on a zinc plated steel substrate were developed to meet such needs as disclosed in Japanese Patent Application Kokai Nos. 108292/1982 and 224174/1983.
These proposals are intended to provide high corrosion resistance by coating a zinc plated steel substrate with a coating composition containing a resin and a water dispersed silica sol. However, the use of water dispersed silica sol gives rise to several problems including (1) limitation of the available type of organic resin binder which has to be compatible with the silica sol, (2) the presence of residual water-soluble components in the coating which allow water to penetrate into the coating during subsequent chemical conversion treatment so that chromium in the underlying chromate layer can dissolve out to contaminate the environment, (3) potential separation of the coating during alkali degreasing which can lead to a loss of corrosion resistance, and (4) poor adherence of the coating in that water can penetrate underneath the coating upon exposure to a corrosive environment, allowing the water soluble components to dissolve therein and exhibit high alkalinity cleaving the interfacial bond between the coating and the chromate. These problems are essentially derived from the use of water as the solvent for the coating compositions.
One solution is disclosed in Japanese Patent Application Kokai No. 22637/1988 which uses a coating composition comprising a hydrophobic silica obtained by subjecting silica on its surface to organic substitution in an organic solvent and an epoxy resin having a primary hydroxyl group and a basic nitrogen atom added thereto. Although compatibility is maintained between silica sol and the organic resin and improved adherence after coating is achieved, this coating has no ability to hold corrosion products stably because of the absence of a free silanol group on the silica surface. Therefore, corrosion resistance is materially low. The use of existing silica sol of either a water or organic solvent system adversely affects weldability.
Further, Japanese Patent Application Kokai Nos. 35798/1988 and 65179/1989 discloses addition of dry fumed silica to organic solvents Although corrosion resistance and weldability are improved, the use of fumed silica not only leads to a substantial increase in viscosity of the coating composition which can interfere with control of the coating weight upon its application, but frequently causes aggregation during blending of the coating composition so that the composition cannot be applied.